The Long QT Syndrome (LQTS) is a group of diseases in which patients have prolongation of the QT interval on the electrocardiogram (ECG). This interval represents repolarization of the heart. These patients are at risk of death due to a syndrome that has a genetic basis in most individuals. Genetic typing can identify patients and family members at risk of development of this syndrome. This is important because therapy is available which can effectively treat this problem and significantly reduce the mortality. This protocol will study patients with suspected LQTS and their families by performing a medical history, and ECG and a 24-hour Holter monitoring. In addition, blood will be drawn for analysis of the genetic structure. Subjects will also under go a noninvasive test called T wave alternans analysis, which involves wearing adhesive electrodes for approximately 30 minutes during rest and mild exercise, while a computer detects and quantifies subtle abnormalities of the T wave.